Achieving the objectives of miniaturization and higher packing densities continue to drive the semiconductor manufacturing industry toward improving semiconductor processing in every aspect of the fabrication process. Several factors and variables are involved in the fabrication process. For example, at least one and typically more than one photolithography process may be employed during the fabrication of a semiconductor device. Each factor and variable implemented during fabrication must be considered and improved in order to achieve the higher packing densities and smaller, more precisely formed semiconductor structures.
In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist, and an exposing source (such as optical light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the photoresist mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through the photoresist mask causes a chemical transformation in the exposed areas of the coating thereby making the image area either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer. The resulting pattern image in the coating, or layer, may be at least one portion of a semiconductor device that contributes to the overall structure and function of the device.
With the advancement of semiconductor fabrication techniques toward smaller, more densely packed features comes increased demands for processing with shorter wavelengths of light and thinner photoresist materials. For example, in order to achieve smaller feature dimensions, wavelengths in the extreme ultraviolet (EUV) range of about 5 to about 25 nm are being employed in association with EUV masks. Such EUV masks can then be used to transfer images to underlying layers of material to form structures characterized by smaller dimensions.
Typically, chrome and nitride are used to form EUV masks. However, these are reflective materials and thus, are more difficult to produce precisely sized as well as smaller sized features therein. As a result, maintaining the precision and accuracy as well as achieving the smaller size dimensions are critical in the manufacture of EUV masks.
Conventional control methods are typically limited to end-point techniques where by inspection of the mask occurs only after the EUV mask has been fully made. These methods are often slow, inefficient and costly and not practical for the current performance demands and high quality standards of today's users and consumers.